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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

.

428 USER COMMENTS

I have an older existing cement floor in my basement that seeps water at the walls, where, if left still, will puddle because the floor is not level. Is it possible to pour a 2 inch slab on top of the old slab, with a 1 inch gap around the walls (for the water to run into a sump pump?). I do not use the basement for any living space..it is for winter wood storage only, but I am tired of walking in a wet basement. Right now, with the cold winter we are having, the basement floor is dry, and would be a good time to add another slab over the existing one. can it be done?? thank you for your time. sincerely, Michael.
- michael
Tuesday, February 24, 2015 at 11:43 AM

Terry, you must remove the paint first, concrete will not adhere to a painted surface. The use of rebar to join the two slabs is also suggested.
- Chris Technical Services
Wednesday, February 11, 2015 at 12:10 PM

Dean, by correcting the slope in the garage floor it will help eliminate the puddling issues. Once you have corrected your slope, a waterproof sealer is recommended to help protect the concrete and prevent moisture from getting thru. Make sure to select a sealer that is a wear topping surface and provides traction.
- Chris Technical Services
Wednesday, February 11, 2015 at 12:06 PM

We want to expand our kitchen using some of our garage. We live in Florida and have no basement. The garage floor is concrete and has been painted and we want to add a slab of concrete on to it. Do we have to remove the paint on that section? What is the best way put the new concrete over the painted concrete
- Terry
Tuesday, February 10, 2015 at 11:14 AM

I have a garage that has been escavated below and a storage area beneath it. During the winter months the garage floor gets wet and it ultimatley leaks down into the storage area. I think in part it is because the garage floor is not sloped to the garge doors and the water puddles. I use a wet vac to pick it up as much as possible and I installed large rubber floormats covering the entire floor but it still leaks. Is there something I can to to top the floor, seal it and slope it to the garage doors?
- Dean
Monday, February 9, 2015 at 1:56 PM

Douglas, the tar adhesive must be removed first before a concrete material is installed. With "cut-back adhesives" which is often a tar like material, removal may need to be done mechanically so you reach stable concrete that the new material can bond to.
- Chris Technical Services
Thursday, February 5, 2015 at 11:02 AM

We are rennovating our house and replacing the old wood floors with new bamboo floors. The old floors where sitting on 2x4's rather than a slab of concrete. A tar adhesive was used under those 2x4's. We cleared up all the wood and now need to pour new cement to level the floors first before adding the new bamboo floors. My question is, do I need to clean up that tar before pouring cement? If so, what is the best way to do it? I tried scrapeing it off but after 2 hours, I knew it was going to be a long process. Please advise if possible. Thanks in advance, Douglas
- Douglas
Monday, February 2, 2015 at 11:19 AM

Cindy, unfortunately you will have to mechanically remove all the sealer down to a solid substrate before using a resurfacing material. A slurry is only used to prime a surface for another material. Portland, color, bonding agent, and water will not have any strength at all. We suggest once the surface is prepped use Sakrete Top ‘N Bond to resurface the counter top. Top ‘N Bond is a polymer modified material that can be used from ½” down to a feathered edge. You may also use cement color with this product.
- Chris Technical Services
Tuesday, January 13, 2015 at 10:15 AM

I have a sealed concrete countertop (3 1/2 years old) with a gross amount of voids, bug holes, and pin holes. Do I need to remove the penetrating sealer prior to applying a slurry comprised of Portland cement, a bonding agent, color pigment, and water?
- Cindy
Tuesday, January 13, 2015 at 3:14 AM

Chez, I suggest using an epoxy because of its strength and adhesion. Sakrete does not manufacture a product that will work for this type of repair.
- Chris Technical Services
Tuesday, December 16, 2014 at 10:55 AM

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